Published 4:00 am, Friday, July 29, 2005

2005-07-29 04:00:00 PDT Washington -- The variety of species in the world's oceans has dropped by as much as 50 percent in the past 50 years, according to a paper published today in the journal Science.

A combination of overfishing, habitat destruction and climate change has narrowed the range of fish across the globe, wrote biologists Boris Worm and Ransom Myers of Dalhousie University in Nova Scotia and three other scientists. In some areas, such as the ocean off northwest Australia where a wide variety of tuna and billfish used to thrive, diversity has declined precipitously.

"Where you used to put out a fishing line 50 years ago and catch 10 species, now you catch five species for the same amount of effort," Worm said in an interview Thursday. "That's a recipe for ecological collapse and disaster."

The study, which marks the first worldwide mapping of predatory fish diversity, identified five remaining spots in the world that still have a rich variety of species, two of them in U.S. waters. The spots include areas off the east coast of Florida, south of Hawaii, near Australia's Great Barrier Reef, near Sri Lanka and in the South Pacific.

"These areas are really of global significance," Worm said. "It's really important to protect them now, because 20 years from now they may not be there. "

The total catch for tuna and billfish has increased as much as tenfold over the past half-century, they found, prompting fish diversity to plummet. Overfishing is the main factor in these species' decline, Worm said, as well as for other fish caught inadvertently.

"That's what driving the pattern," he said.

Water temperature's effect

But in an example of how shifts in temperature can also affect diversity, the study found that in the Pacific, the variety of fish expanded when the weather pattern known as El Niño swept in and brought warmer surface water, but then contracted when temperatures dropped.

Predatory fish appear to like medium temperatures around 77 degrees Fahrenheit, Myers said: "Like Goldilocks and the three bears, ocean animals don't like it too hot or too cold -- they like it just right."

To do the study, Worm and Myers -- along with Marcel Sandow and Andreas Oschlies of Germany's Leibniz Institute for Marine Science and Heike Lotze of Britain's National Oceanography Centre -- used data from Japanese long-line fisheries going back to the 1950s, which they cross-referenced with scientific observer data from the United States and Australia.

Species indicators

The researchers determined that tuna and billfish are indicators of wider ocean diversity, and that these species are disappearing in many areas. Midsize predators -- snake mackerel and pelagic stingrays -- are taking their place.

Worm compared the diminishing range of species to a poorly distributed stock portfolio that's ill-equipped to respond to economic and environmental shifts.

"As (fishing) markets change, as the climate changes, you have nothing to fall back on," he said.

Myers said international authorities need to ban fishing in ecologically valuable sites if they want to preserve them.

"We need protected areas in the open ocean," he said. "The open ocean is still open access."